Processes for preparing thick films by electrophoresis have been known. Preparation of films by electrophoresis comprises suspending a powder of the starting material in a solvent system (liquid medium), applying an electric field to the suspension using, as the cathode, a substrate plate on which a film is to be formed and thus causing the charged particles in the solvent system to be deposited on the surface of the substrate by electrically attracting said particles.
Although preparation of films by electrophoresis is practised for formation of films of various compositions, there is known no case wherein films of solid electrolytes were prepared by electrophoresis. Further, known processes for preparation of films by electrophoresis are practised with electroconductive or semielectroconductive substrates, and films cannot be formed on the surface of oxide substrates which are strong insulators. Also, no attempt has been made to prepare laminated films. That is, only single layer films were made by the conventional method.
We sought to develop an improved method of electrophoretic formation of films and found that the property of the formed film is greatly influenced by the composition of the solvent system used. Thus we conducted a study to find a suitable solvent system and have found a solvent system of a specific composition which enables formation of films on oxide substrates, which has been difficult, and the preparation of excellent thick films of solid electrolytes can be prepared.